For proper operation of an internal combustion engine, it is required that in all operating conditions, an ignitable mixture shall be present at the spark plug, and in the space containing the electrodes. In certain operating conditions, this requirement may not be fulfilled, as for example, when operating at low loads, where the gas in the electrode space contains a high proportion of exhaust. In operation of the engine with an excess of air which is desirable for low consumption and clean exhaust, it is likewise not assured that there will always be an ignitable mixture at the spark electrodes.
In the case of revolving piston engines, a further problem arises through the need to employ spark plugs of high heat rating that will withstand the thermal stress of full load. Spark plugs of high heat rating have a minimum of heat absorbing surface and an electrode space of minimum size, with the result that at partial load the gas in the electrode space is exchanged for fresh gas in small part only, thus containing a high proportion of exhaust and consequently, is not very ignitable. Furthermore, there are no major gas currents in the electrode region, so that any deposits formed are not blown away. This may cause spark plug failure by short circuit unless special ignition systems with a rapid voltage rise are used.